Illustrated herein are methods of preparing encapsulated bichromal balls and the resulting balls produced thereby. The methods find particular application in conjunction with the use of such balls in display devices, such as photochromic displays, and substrates such as “electric paper”, and will be described with particular reference thereto. However, it is to be appreciated that the embodiments are also amenable to other like applications.
Bichromal rotatable elements, such as bichromal balls, or beads as sometimes referred to in the art, are tiny spheres, such as micron-sized wax beads, which have an optical and an electrical anisotropy. These characteristics generally result from each hemisphere surface or side having a different color, such as black on one side and white on the other, and electrical charge, i.e., positive or negative. Depending on the electrical field produced, the orientation of these beads will change, showing a different color (such as black or white) and collectively create a visual image.
In this regard, the spherical particles are generally embedded in a solid substrate with a slight space between each ball and the substrate being filled with a liquid so that the balls are free to rotate in a changing electrical field, but can not migrate from one location to another. If one hemisphere is black and the other is white, each pixel can be turned on and off by the electrical field applied to that location. As a result, each pixel can be individually addressed, and a full page image can thus be generated.
For example, reusable signage or displays can be produced by incorporating the tiny bichromal beads in a substrate such as sandwiched between thin sheets of a flexible elastomer and suspended in an emulsion. The beads reside in their own cavities within the flexible sheets of material. Under the influence of a voltage applied to the surface, the beads will rotate to present one side or the other to the viewer to create an image. The image stays in place until a new voltage pattern is applied using software, which erases the previous image and generates a new one. This results in a reusable signage or display that is electronically writable and erasable.
Conventional display devices, components for display devices, and the manufacture of such display devices and their components are described generally in Sheridon, U.S. Pat. No. 5,604,027; Jacobson et al., U.S. Pat. No. 5,961,804; Jacobson et al., U.S. Pat. No. 5,930,026; Albert et al., U.S. Pat. No. 6,067,185; Crowley et al., U.S. Pat. No. 5,262,098; Sheridon, U.S. Pat. No. 5,344,594; and, Stefik, U.S. Pat. No. 5,723,204, the contents of which are incorporated entirely herein by reference.
Gyricon or bichromal balls or beads utilized in these displays are typically produced by a spinning disc method and generally exhibit a wide size distribution, i.e., from about 50 to about 200 microns. The resolution of the Gyricon display is dependent on the bead size. The Gyricon beads used in current displays are from about 75 to about 110 microns (μm). However, for many applications, it is necessary to use bichromal beads of much smaller size in order to achieve higher resolution as well as lower switching voltage. It has been found extremely difficult to produce smaller beads while still maintaining acceptable characteristics such as bichromality, complementarity, sphericity, etc., and manufacturing yield.
Current Gyricon displays use a swollen elastomeric sheet in which Gyricon beads are dispersed. Recent investigation has shown that by encapsulating the Gyricon beads within an oil-filled capsule, the need to contain the swelling fluid as well as the need for the costly elastomer can be eliminated. However, the encapsulating process also produces some empty capsules, which must be completely removed in order to prevent degradation of the optical contrast of the display. This further adds to the cost and complexity of manufacture. Accordingly, there is a need for an improved method for encapsulating bichromal balls, without the problems or degree of such problems, as currently known.
The use of gelation or gelling techniques in the formation of bichromal balls is not generally known. Although the art refers to gel substrates for retaining bichromal balls, i.e. U.S. Pat. Nos. 5,604,027; Re 37,085; this approach merely employs a gelled substrate which houses a bichromal ball. That is, this description is not concerned with the actual formation of the bichromal ball itself. U.S. Pat. Nos. 6,488,870 and 6,492,025 describe forming a shell about a bichromal ball, which in certain embodiments, may be formed by certain gelation techniques. However, these approaches are not relevant to a process of forming the actual bichromal ball or bead.